@muntasirms I've done several experiments on Fe/Mn and Zn/Mn chemistries. The problem with Mn2+ is the formation of the solid MnO2 phase and the presence of the metastable Mn3+. Forming solid MnO2 poses a non-trivial constraint on the battery, as it limits deposition per area to around a few mAh/cm2, very impractical for a flow battery, furthermore, Mn3+ formation causes MnO2 to form away from the electrode (as it disproportionates into Mn2+ and MnO2), causing some Mn to become lost around the battery system.
A possibility is to try to stabilize Mn3+ somehow (for example with Mn-EDTA), but the main issue is that even this stabilized Mn3+ is unstable and eventually self-degrades by oxidizing the chelate around the Mn atom. I tried creating a flow battery system with Fe-DTPA/Mn-EDTA, which has a max solubility of around 0.5M, but the system did not cycle due to the Mn-EDTA being too unstable. There are a few posts on my blog about this. The oxidized Mn-EDTA is also quite sensitive to pH, so it is hard to create conditions under which it is stable. Mn3+ can also be stabilized with HCl+H2SO4, but only at very low concentrations (there's a paper on using this in a flow battery, but only very low capacities are achieved).
Another possibility is to stabilize MnO2 as nanoparticles in solution. This can be achieved through the use of TiO2+ in sulfuric acid (using titanyl sulfate). Such systems are quite harsh from a chemical perspective, so I haven't tested them at all (I don't want to run a 3M sulfuric acid system containing reactive Ti compounds). You can read more about this system here (https://www.sciencedirect.com/science/article/pii/S0378775322000209). This is one of the most interesting and potentially viable Mn chemistries out there although only reaching around 17Wh/L.
Honestly Mn based systems are best suited for static batteries, where the formation of the MnO2 and Mn3+ phases is less problematic.
